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Sandy and Chris stand on the edge of a cliff and throw identical mass rocks at the same speed. Sandy throw her rock horizontally while Chris throws his upward at an angle of 45° to the horizontal. (i) Which one has a higher speed : Sandy’s thorwn rock at the launch or Chris’s rock when it reaches its highest point in its flight? (ii) Which one (sandy’s rock or Chris’s rock has the higher speed right before hitting the ground? : Explain carefully your answer.

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Answer:

i) Sandy, ii) Sandy's rock.

Explanation:

i) Both rocks experiments a parabolic motion with the difference that the rock launched by Chris was made against gravity. The velocities are, respectively:

Sandy

[tex]v_{s} = v[/tex]

Chris:

[tex]v_{c} = v\cdot \cos 45^{\circ}[/tex]

[tex]v_{c} = \frac{\sqrt{2}}{2}\cdot v[/tex]

Sandy reports a higher speed.

ii) Sandy's rock reports a speed higher than Chris's rock, since launching upwards means that gravity decelerates the vertical velocity of the rock until rest and before accelerating.

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